Biot Lecture 2004

Abstract: Living bones adapt their structure to meet the requirements of their mechanical environment. These adaptations require a cell-based mechanosensing system with a sensor cell that perceives the mechanical deformation of the mineralized matrix in which the cell resides.

One of the most perplexing features of this mechanosensory system in bone is the very low strain level that a whole bone experiences in vivo compared to that needed to produce a response in cells. The amplitudes of the in vivo strains generally fall in the range 0.04 to 0.3 percent for animal locomotion and seldom exceed 0.1 percent. These strains are nearly two orders of magnitude less than those needed (1% to 10%) to elicit biochemical signals necessary for communication of the sensing cells with the cells that deposit and resorb bone tissue. There is a paradox in the bone mechanosensing system in that the strains that activate the bone cells are at least an order of magnitude larger than the strains to which the whole bone organ is subjected.

A hierarchical model ranging over length scales that differ by 9 orders of magnitude, from the subcellular level to the whole bone level, is used to resolve this paradox. Using this Biot poroelasticity based model it is possible to explain how the fluid flow around a bone cell process can lead to strains on the cell process structure that are two orders of magnitude greater than the mineralized matrix in which the cell resides.

Stephen C. Cowin is a City University of New York Distinguished Professor in the Departments of Biomedical and Mechanical Engineering at City College. His principal research interest is the mechanics of materials, particularly in determining the influence of microstructure on the gross mechanical behavior of granular, composite, and biological materials. He is the Director of the New York Center for Biomedical Engineering at the City University of New York and Adjunct Professor of Orthopaedics at the Mt. Sinai School of Medicine. Before taking up his position at City College in September 1988 he was the Alden J. Laborde Professor of Engineering in the Department of Biomedical Engineering at Tulane University. He received his BES and MS in Civil Engineering from Johns Hopkins University in 1956 and 1958, respectively, and his Ph.D. in Engineering Mechanics from the Pennsylvania State University in 1962. After one year on the faculty at the Pennsylvania State University, he began a 25-year association with Tulane University in 1963. He was the recipient of three awards from the ASME, the Best Paper Award (1992), the Melville Medal (1993) and the H. R. Lissner Medal (1999) for contributions to biomedical engineering. In 1994 he received the European Society of Biomechanics Research Award. In 2004 he was elected to the National Academy of Engineering and he also received the Maurice A. Biot medal of the ASCE.

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